An extrusion coating process generally includes an extruder slot (cast) die mounted in a position above a substrate to be coated. The substrate moves past the die. Gravity causes a molten stream of the extrusion coating material to fall onto the substrate. The thickness of the extrusion coating is controlled by the rate of output from the die and the rate of speed at which the substrate is moving beneath the die. The substrate can be moved beneath the stream of extrusion coating material flowing from the die in various ways. In certain methods the substrate material is moved on a conveyor belt beneath the die. Other means include grabbing the substrate material and moving it on driven rollers, beds or the like. Still other coating methods involve transporting the substrate over a roller or through a set of nip rollers adjacent the slot die. The coating material extrudes from the slot die onto the substrate at the nip. The nip rolls add pressure to the substrate and coating material at the interface to aid in achieving a bonding between the coating material and the substrate.
These extrusion methods require that the coatings have a sufficient thickness such that the coating material completely coats the substrate and that there are no spaces or gaps in the coating material.
While it would be desirable to apply an extrusion coating to such type of film, various difficulties occur when using the currently known coating technologies. Until the present invention, both the thermal energy of the extrusion coating systems and the compressive energy of the nip roll systems made it virtually impossible to achieve good bonding between the apertured substrate and any coating material applied thereto without causing damage to the apertures or the substrate itself. This is of particular concern when a thin coating is desired to be applied to the substrate.
Previously, attempts to apply a coating material onto thin apertured three-dimensional materials which are particularly sensitive to excessive thermal loads have not met with success. The apertured three-dimensional material does not have sufficient mass to resist distortion under the required thermal load necessary to achieve a good bond between the extrusion coating material and the three-dimensional material.
It is therefore an object of the present invention to provide an improved method for applying a layer of coating material to apertured, three-dimensional thermally sensitive substrates.
It is still another object of the present invention to provide a fluid impervious article comprising an extrusion coating material bonded to a thermally sensitive polymeric layer having multiple apertures.
It is still another object of the present invention to provide a substantially fluid impervious article suitable for use as a backsheet for a disposable absorbent and used article such as diapers, catamenial pads, surgical dressings and the like.